That's just not what traditional QM says. Traditional QM separates quantum systems (microscopic) from measurements devices (macroscopic). A measurement occurs when the measurement device interacts with the quantum system. You do QM by predicting the results of these measurements. Consciousness does not enter into it.

So why doesn't a measurement and/or decoherence occur when double slits or a half-silvered mirror interact with a passing particle? Are they not macroscopic objects which interact with the quantum information?

Perhaps it's only a "measurement" if the (alleged) particle has nowhere else to go after the interaction. But how does the (alleged) particle know which macroscopic interactions are terminal and should be counted as "measurements" and which are part of the rest of the experiment?

There is no answer to this in QM. You can calculate the probabilities and you will get predictable answers, but there are still >20 interpretations of what is really happening, and they all disagree with each other in important ways.

Until you get information from the measurement, it didn’t happen.

Can it be considered as until it causally affects something else (the observer), its state is not defined? Isn't it a causal relation that collapses the superposition?

Why from all people?

What if there is a person in an lab which is isolated from the rest of the people?

Wigner’s friend?

Then he will certainly be able to say a measurement happened.

That’s something only conscious beings can do.

But then isn't Wigner's friend also in a superposition until Wigner talks to them?

Traditional QM does not allow Wigner’s friend to be in superposition.

Until he talks to them, how can Wigner tell? ;-)

Wigner can’t tell, but the friend knows. If he thought he was in a superposition he would believe in many worlds.

But shouldn't every friend in every possible state observe a single state that's consistent with their own internal state?

The friend only exists in many worlds from Winger's perspective, and only as long as their state has no causal effect on Wigner's own state.

What's wrong with many worlds?

It’s not what this thread is about.

I guess I got confused at "If he thought he was in a superposition he would believe in many worlds."

Do you mean "he can either think he can be in superposition or he cannot be, if he thinks he can be then he would believe in many worlds, which is incompatible with 'traditional' QM, which thus implies 'Traditional QM does not allow Wigner’s friend to be in superposition.'" ?

I don't think the Traditional QM ever explicitly disallowed a person to be in superposition; being in superposition it's just not something "decent people" do, I suppose.

Thus we needed to find other ways to come to terms with what we observe. But the math is the same. The rest is only metaphors that help us reconcile what we observe with how we feel inside.

Thus, many worlds is not antithetic to traditional QM.

Do you think that many quantum physicists during the 21st century accepted the possibility of living in a splitting multiverse?

I don't think that's historically accurate. If you read about how Everett's ideas were treated it's pretty clear traditional QM is antithetic to many worlds.

Sure, I agree with you that most people vehemently rejected that interpretation.

But the QM theory is not its interpretation.